The present invention relates to a heat dissipation arrangement for chip modules on multilayer ceramic substrates,
In a conventional heat dissipation arrangement for modules in which one or more chips are arranged on a multilayer ceramic substrate, the one or more chips, in particular the one or more power ICs, are arranged on thermal passages, or xe2x80x9cthermal viasxe2x80x9d. These thermal passages are a regular arrangement of holes, or hole arrays, that are punched through all circuit layers and filled with silver paste. Transferring heat from the power ICs through the thermal passages to the metal base plate located beneath the multilayer ceramic substrate is not very effective. The passages punched through all circuit layers take up too much surface area that could otherwise be used by other printed conductors on the circuit layers. This reduces the packing density.
The conventional technical solution using thermal conductive modules (TCMs) developed by IBM conducts the heat output by the ICs very effectively upward at right angles to the substrate. However, this solution is too expensive for many applications.
Generally, heat pipe structures are devices that can be used to transfer quantities of heat from one point where heat is generated to another point. With a suitable design, the effective heat transport of this arrangement is tens of times higher than that of the best metal heat conductors. A description of how the heat pipe structures work is provided, e.g., in xe2x80x9cThe heat pipexe2x80x9d by Y. Eastman is Scientific American, 5/68, pages 38 to 46.
The principle of heat pipes can be described as follows: a suitable fluid with a high latent evaporation heat evaporates in the hot area of the arrangement. The pressure produced by evaporation drives the steam to the cold part of the arrangement. There the steam condenses, forming the liquid phase, and gives off the transported heat. The liquid condensate is then returned to the evaporation point. This establishes a circuit.
The heat dissipation arrangement for chip modules according to the present invention, has the advantage over conventional arrangements in that the principle of heat pipe is used in a miniaturized form for the effective cooling of power ICs that are mounted on relatively poorly conductive multilayer ceramic substrates, in particular glass ceramic multilayer circuits. The heat is transferred to the metal heat sink much more effectively, while retaining the packing density and circuit flexibility, since the amount of surface area needed is minimized.
According to the present invention, this is achieved, in principle, by mounting the multilayer ceramic substrate on a metal heat sink; providing thermal passages, in particular in the form of a hole pattern or array, in the top layer of the multilayer ceramic substrate, in the region of the chip to be mounted; providing a cavity functioning as an evaporation chamber in the layer of the multilayer ceramic substrate directly beneath the top layer, in the region of the chip to be mounted; providing a trough-shaped recess functioning as a condenser in the metal heat sink, in the region of the chip to be mounted; and providing, in the layers of the multilayer ceramic substrate located between the evaporation chamber and the condenser, in the region of these chambers, a series of large steam channels and small condensate channels, the latter functioning as capillaries, which interconnect the two chambers.
According to an advantageous embodiment of the present invention, a plurality of chips, in particular power ICs generating a considerable amount of waste heat, can be mounted on the multilayer ceramic substrate.
According to a further advantageous embodiment of the present invention, each trough-shaped recess functioning as a condenser and located in the multilayer ceramic substrate is provided with a closable opening for filling a heat-conducting medium.
According to an especially advantageous and easy-to-manufacture embodiment of the present invention, the large steam channels and the small condensate channels are designed as straight channels between the layers of the multilayer ceramic substrate.
According to an embodiment of the present invention as an alternative to this embodiment, which can be useful in some applications, in particular with regard to the space requirements and conductor arrangement, the small condensate channels are designed, at least in part, as angled channels, with at least a portion of these channel provided on the same plane as one or more layers of the multilayer ceramic substrate. The portion of the condensate channels provided on the plane of one or more layers can be produced in different ways. They can be produced by printing, with these portions of the channels being printed with carbon paste which is later burned off to produce the partial channels, or they can be produced by punching, milling, or embossing the channel portions in the unfired ceramic material, in particular in the ceramic tape. In an advantageous embodiment of the present invention, the large steam channels have a diameter of around 1.5 mm, and the small condensate channels have a diameter of around 0.1 mm.
In a further an embodiment embodiment of the present invention, which can help improve conduction of the condensate, the small condensate channels are filled with a porous ceramic powder or with metal frit which remains unsintered at the firing temperatures used.
To prevent direct evaporation of the condensate, according to a further advantageous embodiment of the present invention, the large steam channels can have, in the region of the condenser chamber provided in the metal heat sink, a diameter or a passage area that is smaller than the passage area of the steam channel in the more distant layers of the multilayer ceramic substrate.
In a an embodiment embodiment of the present invention, a fluid having as high an evaporation heat as possible at a suitable evaporation temperature, as well as a high surface tension and wetting angle for the capillaries, in particular alcohols and hydrocarbons, is used as the heat-conducting working fluid.
According to a further advantageous embodiment of the present invention, the one or more individual chips can be attached to the multilayer ceramic substrate using a suitable heat-conducting adhesive, and/or the multilayer ceramic substrate can be attached to the metal heat sink using a suitable heat-conducting adhesive.